1. Field of the Invention
The present invention relates to a system for controlling idling speed in an internal combustion engine for a vehicle provided with an automatic transmission device, and capable of decreasing shock generated when the automatic transmission is manually operated to attain a shift down operation for engine braking to decelerate a vehicle.
2. Description of the Related Art
To attain an engine braking for decelerating a vehicle provided with an automatic transmission, usually a shift lever must be manually moved from a D position to a 2 or L position, or occasionally by switching off an over-drive switch while the throttle valve is fully closed. In this case, a large shock is generated when the automatic transmission carries out a change from a higher ratio gear to a lower ratio gear because a friction engagement unit (clutch or brake) for the lower transmission stage can transmit a torque which is excessively larger than a value of the torque from the engine itself, so that the speed of the engagement of the friction engagement unit for the lower ratio gear becomes too fast, and thus an uncontrolled change in torque in the output of the transmission device is generated.
To prevent an excessive increase in the speed of the engagement of the friction engagement unit for a low ratio gear, a simple solution can be found wherein a hydraulic pressure for the friction engagement unit is decreased to suppress any shock that will be generated during the engine braking operation. This solution, however, also prolongs the time necessary to complete a shift operation when the vehicle speed is high, causing the friction element of the friction engagement unit for the high ratio gear to be easily damaged.
To overcome these difficulties, it is possible to change the level of the hydraulic pressure for the friction engagement unit in accordance with the speed of the vehicle. With this solution, however, the construction of the hydraulic control circuit becomes complicated.